1. Field of the Invention
The invention is related to the field of communications, and in particular, to IMS gateway systems and methods of operating IMS gateway systems.
2. Statement of the Problem
As set forth in the 3rd Generation Partnership Project (3GPP), an IP Multimedia Subsystem (IMS) provides a common core network having access-agnostic network architecture for converged networks. Service providers are accepting this architecture in next generation network evolution. The IMS architecture is initially defined by the 3GPP to provide multimedia services to mobile subscribers over an Internet Protocol (IP) network. IP networks have become the most cost savings bearer network to transmit video, voice, and data. IMS uses the advantage of IP networks to provide multimedia services for IMS subscribers on an IMS platform. The signaling used within IMS networks is Session Initiation Protocol (SIP). IMS defines the standard SIP interface between application servers, the IMS core network (CSCF), the IMS subscriber, the IMS database (HSS), and IMS billing elements. These standards can reduce the network integration costs and let the subscriber enjoy more stable services.
On the IMS platform, the traditional supplementary services, such as call forwarding, conferencing, and call waiting are available for IMS subscribers. Also, many new data services, such as instant messaging, video calls, video on wait, and web-based services, will also be available for the IMS subscribers.
Providing efficient IMS online charging for operator revenue generation is important to the successful deployment of IMS networks. Several 3GPP technical specifications describe online charging for IMS networks. For instance, the 3GPP TS 32.200 specification describes an online charging system (OCS) having a session-based charging function. The OCS is coupled to a serving-call session control function (S-CSCF) through an IMS service control (ISC) interface. The S-CSCF controls a call session for a calling party or a called party and communicates with the OCS over the ISC interface to provide online charging for the call session. However, an ISC interface is a service interface that does not support online charging. Therefore, in order to use the ISC interface between the S-CSCF and the OCS for online charging, additional functionality would unfortunately need to be added to the OCS.
To avoid overloading the OCS with additional functionality and to keep the online charging architecture consistent, the interface between the S-CSCF and the OCS may be changed to support online charging instead of adding functionality to the OCS. One option for an interface that supports online charging is to extend the ISC interface to allow for charging mechanisms. The ISC interface would then be both a service interface and a charging interface. Unfortunately, using the ISC interface as a hybrid service/charging interface may not be acceptable for standardization desired by the 3GPP.
Another option is to use the Ro interface instead of the ISC interface because the Ro interface already supports online charging. The 3GPP TS 32.296 specification suggests using the Ro interface for online charging by introducing an IMS gateway function that acts as a gateway between the S-CSCF and the OCS.
FIG. 1 illustrates an IMS online charging architecture 100 as suggested by the 3GPP in the prior art. Online charging architecture 100 is described in 3GPP TS 32.240 and 32.260. Online charging architecture 100 includes IMS gateway function 102, an S-CSCF 104, and an OCS 106. OCS 106 includes a session-based charging function (SBCF) and an event-based charging function (EBCF). The session-based charging function is responsible for online charging of network/subscriber sessions, such as voice calls or IMS sessions. The event-based charging function performs event-based online charging (also referred to as “context charging”) in conjunction with any application servers.
IMS gateway function 102 communicates with S-CSCF 104 over the ISC interface 105 and communicates with OCS 106 over the Ro interface 107. For online charging communication between S-CSCF 104 and the session-based charging function in OCS 106, S-CSCF 104 does not trigger online charging events and thus does not include a Charging Trigger Function (CTF). Instead, the ISC interface 105 is employed by the S-CSCF 104, implying that online charging is transparent to S-CSCF 104 and appears like any other service controlled by a SIP application server. Therefore, if support for Ro-based online charging is required, a special CTF is needed in order to mediate between the Ro-based session-based charging function and the SIP-based service control. This role is taken by IMS gateway function 102, which translates between SIP session control towards S-CSCF 104 and the Ro credit control towards OCS 106.
Unfortunately, the 3GPP specifications do not describe how to use the IMS gateway function for online charging. The specifications also do not resolve how the ISC interface, the Ro interface, and the S-CSCF would function together, and how the IMS gateway function would operate to provide budget control for online charging.